Or to give another more common example: If you delete your contact list,
I likely might find again by just searching for 0x6f726956 at the device
level (assuming you've stored a contact in that list with the same
surname as yours.
And, because I've only mentioned in a different thread, now think at the
problem that nowadays storage is often fixed (soldered) to devices which
don't offer a way to delete the whole storage. You might have luck if
the contact list in question was stored in some encrypted part, but that
presumes that the key for that encrypted part isn't somehow stored on
the same device too. Which unfortunately isn't always the case (maybe
because of usability). And ...
That's why I think filesystems should offer a way to really delete
files. Most people would be happy, even if filesystems won't delete
stuff at military security levels and would disregard all the cases when
they couldn't make sure that stuff is really deleted.
To conclude, most people would be already happy if the most trivial case
would be handled right and not just by marking files as deleted but
leaving the contents intact.